1. Field of the Invention
The present invention relates to a correlator that determines the correlation between an OFDM signal, which has an effective symbol period and a guard interval in which part of the effective symbol signal has been copied, and a delay signal obtained by delaying the OFDM signal. The present invention also relates to a demodulation device that includes the correlator. The present invention in particular relates to a correlator and a demodulation device including the correlator, capable of deriving autocorrelation from which influence of undesired power has been removed even when undesired power is incorporated in the received signal band.
2. Description of the Related Art
In recent years, the orthogonal frequency division multiplexing (OFDM) modulation method has been used as a modulation method in digital terrestrial broadcasting.
In the OFDM method, multiple subcarriers with different center frequencies are utilized to transmit symbols. Here, a symbol is a set of data transmitted in one modulation.
One symbol cycle is configured as a result of a guard interval (GI) being added to the effective symbol period. In the OFDM method, as shown in FIG. 9, part of the effective symbol signal serving as the actual demodulation target is copied and inserted between effective symbol signals as a repeated waveform. This allows OFDM to suppress the influence of multipath interference. The interval of this copied waveform is the guard interval.
In the case of demodulating this OFDM signal, the received OFDM signal is digitally converted by an A/D converter, the guard interval is removed, the effective symbol signal is extracted, and the effective symbol signal is demodulated by a fast Fourier transformer (FFT). Namely, as shown in FIG. 8, a correlation value between the received OFDM signal and the signal obtained by delaying the OFDM signal by the length of the effective symbol period is determined. Additionally, a maximum value of a value obtained by integrating this correlation value is extracted, and, on the basis of the timing of this maximum value, the guard interval is removed, the effective symbol period is extracted, and the effective symbol period is demodulated by the FFT (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 11-163824 and JP-A No. 2000-059332).
However, as shown in FIG. 11, sometimes an undesired waveform signal (hereinafter referred to as “undesired power” or “undesired signal”) having particular frequencies and large electrical power may be incorporated in the received signal band. Such undesired power does not have periodicity from copying the identical signal component such as the OFDM signal. Consequently, a rise in the proportion of undesired power causes a greater unintended fall in the correlation relationship. When the correlation value becomes too small, timing synchronization cannot be achieved based on the correlation value. This results in adverse impact on reception quality and sometimes reception cannot be obtained.
In consideration of the above, a technique is proposed to remove the incorporated undesired power by inserting a filter (see, for example JP-A No. 9-321733).
However, in the technique described in JP-A No. 9-321733, only the one OFDM symbol delayed signal is passed through the filter from the two input signals employed for obtaining a correlation. Accordingly, particular noise may remain in the input signal to the correlator, that is not delayed by one OFDM symbol. Consequently, the technique of JP-A No. 9-321733 is not satisfactory for securing robustness to noise with an intense particular frequency component.